GMOS Observing Overheads

For the Phase I observing proposals, that are not using Nod-and-Shuffle and do not require detailed timing, you can use the telescope setup times given below, plus 2 min per exposure to cover readout, filter changes etc. For Nod-and-Shuffle additional overheads apply. These are described on the Nod-and-Shuffle page. The Nod-and-Shuffle overheads should be added to the overheads from the telescope setup and the readout etc.

For more exact calculation of the overheads where these are critical to your observing program (for example time-resolved observations), the detailed listings on this page may be used. The detailed overhead information is also useful for the Phase II planning of your observing program.

Telescope Setup time

The setup times below include slewing to a new target, starting guiding, and accurately centering objects on slits (if appropriate).

Note that the setup time for any spectroscopic target that has already been observed is shorter, since the initial offsets are known and only fine-tuning of the centering is needed. This also applies to fields where a second MOS mask is made from the same pre-imaging data and the same alignment objects are used.

A change in the position angle if already at the target takes 15min. For example, if two long-slit observations of the same target but different position angles are planned, and they can be obtained together, then the overhead for acquiring the target at the second position angle is 15min, instead of 20min.

GMOS configuration times

Below are listed approximate configuration times for various components within GMOS (the exact times depend on the details of the positions between which a particular component is being moved). It is possible to reconfigure GMOS while slewing to a new target, but it is not currently possible to reconfigure GMOS while reading out the detector. This is a limitation of the current software that is used to execute sequences.

Readout times

Below are listed the readout times, including the overhead added by the sequence executor software, which is used to execute sequences.

At present it is not possible to reconfigure GMOS, dither the telescope position or begin setup on the next target during readout.

* For more details on Regions of Interest (ROIs) see the GMOS OT component page .
** The "Central spectrum" ROI corresponds to the central 1024 (unbinned) rows of the detector array in the spatial direction and the full width of the array in the spectral direction.

Telescope offsetting time

The time to offset the telescope as part of a dither sequence is currently approximately 10s (from the time of turning off guiding at one position to guiding at the next position). At present it is not possible to offset the telescope while reading out the detector when these operations are part of a sequence.

Configuration for Calibrations

A portion of the overhead for taking calibrations is the time it takes to move the science fold mirror, which sends the beam either from the sky or from GCAL into GMOS. Again, when using the sequence executor to run sequences, this move cannot be done while reading out the detector array.

With GMOS on side port 5, the relevant science fold moves take about 20 seconds each. Therefore the total overhead to move to and from the calibration position is about 40s. (This does not include the time to actually take the calibrations).

In original form: Isobel Hook
Last update: August 30, 2005; Inger Jørgensen